PROBLEMS OF DISTRIBUTION AND THEIR SOLUTION. 175 



deer, and other quadrupeds, is proved by the fact that these 

 animals are known to have lived and flourished in Europe long 

 before they occurred in America. So that, as Mr. Wallace puts it, 

 "As the theory of evolution does not admit the independent deve- 

 lopment of the same group in two disconnected regions to be 

 possible, we are forced to conclude that these animals have migrated 

 from one continent to the other. Camels, and perhaps ancestral 

 horses," adds Mr. Wallace, " on the other hand, were more abundant 

 and more ancient in America, and may have migrated thence into 

 Northern Asia." The physical difficulties of such a land connection 

 at Behring's Straits or across Baffin's Bay, are not, it may be remarked, 

 by any means insuperable. 



Then, likewise, we must take into account the share which South 

 America, or the Neotropical region, has had in influencing the dis- 

 tribution of life in the New World at large. North America seems 

 in the Post-Pliocene epoch to have been a literal focus wherein 

 Palsearctic life commingled with life from the South. Thus the 

 North American Post-Pliocene deposits give us sloths and other 

 forms of Edentate mammals, llamas, tapirs, and peccaries, all of 

 which are typically South American ; whilst some are identical with 

 living Neotropical species. The bone-caves of South America show 

 us that this region, like Australia, possessed in Post-Pliocene times 

 the same description of quadruped life that now distinguishes it. As 

 giant kangaroos lived in Australia, so gigantic sloths and armadillos 

 lived in South America ; and its chinchillas, spiny rats, bats, and 

 peculiar monkeys were likewise existent then as now. In addition, 

 we find that, as North America possessed its peculiar groups of lower 

 quadrupeds in its tillodonts and other forms, so South America 

 likewise had its special types of life, such as the Macrauchenia^ 

 resembling the tapirs, and the Toxodonts, related at once to the 

 hoofed quadrupeds and to other groups. But, whilst the quadruped 

 immigrations into North America likewise affected South America, it 

 must be borne in mind that the isolation and separation of South 

 America from the northern part of the continent, as indicated by its 

 regional distinctness, must have largely influenced the develop- 

 ment of its own peculiar life just, indeed, as the peculiarities of 

 North America are due to its separation, in turn, from the Palaearctic 

 area. And when we further discover the all-important fact that the 

 fishes on each side of the Isthmus of Panama are identical, the 

 theory of the relatively recent continuity of sea at this point, and 

 the consequent separation of Neotropical from Nearctic land, rises 

 into the domain of fact. Thus we see in North America a region 

 which has repeatedly received and exchanged tenants with the great 

 Europeo- Asiatic continent ; which has, in consequence, developed a 

 close resemblance to the life of the Palaearctic region ; and which 



